THE ELEMENTS OF. MICROSCOPY. 47 
any obliquity will pass ; but this is not so when the oblique ray is 27 
the denser medium. 
This has a very important bearing upon microscopical illumina- 
tion, and I wish you to thoroughly understand it, as many really 
good microscopists have stumbled over this question. It has a 
greater bearing upon the manipulation of objectives than at first 
thought we would imagine. 
Before leaving the subject of refraction, I would like to show 
you one effect of cover glasses upon oblique rays of light, and one 
which should be well studied by all workers with dry objectives of 
wide aperture. I must first call your attention to what I have 
already said repecting total reflection. Now, all rays less than the 
critical angle, I have told you, will pass through the cover glass and 
emerge into air at the surface; rays of greater obliquity will be 
totally reflected. Now, if we produce these air rays backwards we 
shall perceive the object apparently lying in different planes,—the 
more central the ray the deeper will be the focus, while as the ray 
becomes more oblique the object apparently lies nearer the surface, 
until at last as we approach infinitely near 180° it appears to be 
coincident with the surface of the glass. This is a very important 
point in connection with the correction of objectives, and one which 
I shall enter upon in my third paper. 
Thus far we have only considered the passage of light through 
media with parallel faces, such as the ordinary microscopical slides, 
or thin-glass covers; let us now examine its passage through media 
not furnished with parallel faces, but which are inclined to each 
other. That form which will interest us most is the prism—not 
only is this form fer se much used in optical experiments, but it 
may be considered as part of a rudimentary lens. A ray of white 
light upon passing through a prism of glass becomes split up into 
several colours—red, orange, yellow, green, blue, indigo, and 
violet ; or, as some prefer to say, ved, yellow, and violet, and their 
mixtures. Now, this elongated coloured band is called a spectrum, 
and it may interest you to know that the relative size and quality 
of the band depends to a great extent upon the nature of the 
medium through which it passes, upon the quality of the light and 
upon the angles of the prism. ‘The red ray is the least disturbed 
from its position ; or, I may say, the rectilinear path, and the violet 
most ; this disturbance being the general rule. 
I have told you that the nature of the light establishes generally 
the character of the spectrum, and in proof thereof the different 
spectra produced from several sources of flame may be well 
studied. In the ordinary solar spectrum, from the dark-red to the 
violet, you will see also a number of dark lines running vertically, 
which are technically termed Fraunhéper’s lines. The seven 
spectra which you see below that of the solar spectrum, are those 
